A laser is a device that emits coherent electromagnetic radiation by optical amplification based on stimulated emission of photons in a gain medium.
Lasers which utilize air as the gain medium have been extensively studied. See, e.g., R. S. Kunabenchi, M. R. Gorbal and M. I. Savadatti, “Nitrogen Lasers,” Prog. Quant. Electr. 9, 259 (1984). Such lasers typically utilize electronic transitions in N2 that emit UV radiation, such as the C3Πu→B3Πg(v=0→0) transition that lases at a wavelength of 337 nm.
Commercially available nitrogen lasers are typically based on collisional excitation of electrons using electrical discharges. See, e.g., E. T. Gerry, “Pulsed-Molecular-Nitrogen Laser Theory,” Appl. Phys. Lett. 7, 6 (1965); G. G. Petrash, “Pulsed Gas-Discharge Lasers,” Soviet Physics Uspekhi 14, 747 (1972); A. W. Ali, A. C. Kolb and A. D. Anderson, “Theory of the Pulsed Molecular Nitrogen Laser,” Appl. Opt. 6, 2115 (1967); R. T. Brown, “Kinetic processes in a laser-heated helium-nitrogen plasma for use as a uv laser medium,” J. Appl. Phys. 46, 4767 (1975); and W. A. Fitzsimmons, L. W. Anderson, C. E. Riedhauser and J. M. Vrtilek, “Experimental and Theoretical Investigation of the Nitrogen Laser,” IEEE J. Quantum Electron. QE-12, 624 (1976).
Such lasers are typically run at low pressure to facilitate the acceleration of electrons to excitation energies. Nitrogen lasers at atmospheric pressure have also been constructed which require fast, nanosecond discharges. See V. S. Antonov, I. N. Knyazev, and V. G. Movshev, “Output radiation of an ultraviolet nitrogen laser excited transversely in an open air cell,” Soviet Journal of Quantum Electronics 4, 246 (1974). The UV radiation produced by these lasers can be used to detect the presence of chemical and biological agents. However, the UV radiation does not propagate in the atmosphere over distances required for stand-off detection.
A remote UV generation scenario for biological and chemical detection has recently been proposed in which the air molecules are Raman or two-photon pumped to the appropriate excited state. See, e.g., V. Kocharovsky, S. Caemron, K. Lehmann, R. Lucht, R. Miles, Y. Rostovtsev, W. Warren, G. R. Welch and M. O. Scully, “Gain-swept superradiance applied to the stand-off detection of trace impurities in the atmosphere,” Proc. Nat. Acad. Sci. 102, 7806 (2005).
In this scheme, the molecular excitations are achieved via simultaneous action of two synchronized picosecond laser pulses with a frequency difference or sum which is resonant with a transition from the ground state to a vibrationally excited state. Strong emission in the backward direction is generated via swept-gain superradiance. See V. Kocharovsky, supra.
In another work, backscattered fluorescence from N2 molecules and ions has been observed in experiments in which a USPL-generated plasma filament was optically pumped. See, e.g., Q. Luo, W. Liu and S. L. Chin, “Lasing action in air induced by ultra-fast laser filamentation,” Appl. Phys. B 76, 337 (2003).